Softshell reservoirs are commonly used in cardiac surgery to mix venous blood and cardiotomy blood in a heart-lung machine circuit. Prior to the surgery, the blood circuit including the softshell reservoir has to be primed with saline solution so that all air in the circuit is removed, and the circulation through the heart-lung machine can instantly be substituted for the natural circulation. When the patient is switched to the heart-lung machine, the saline solution used for priming mixes with the patient's blood and dilutes it. This is physiologically undesirable not only because it makes the blood supply to the patient temporarily non-homologous, but because it produces symptoms of anemia that keep the patient weak and listless for several days after the surgery until the body rebuilds a sufficient concentration of blood cells.
It is therefore important to minimize the priming volume of the blood circuit in the heart-lung machine. Perfusionists are conscious of that problem and attempt to alleviate it by choosing low prime oxygenators and filters, and by minimizing the length of the tubing interconnecting the components of the blood circuit. A major factor in the priming problem is the softshell reservoir bag. This bag typically holds about 800-1300 ml of fluid. To prime it, the entire bag must be wetted out, thereby requiring a priming volume in excess of 1 liter of saline solution. It would thus be highly desirable to provide a softshell reservoir which has a similar capacity but which requires substantially less priming volume.
Another problem of conventional softshell reservoirs is the occurrence of stagnation areas and incomplete mixing of cardiotomy and venous blood. This is due to the asymmetrical design of conventional softshell reservoirs, in which the cardiotomy and venous inlets are typically on one side of the reservoir's central vertical axis, and the outlet is on the other side. Because both the inlets and the outlet enter the conventional bag in a vertical direction at the bottom of the reservoir, this results in different path legths and configurations for the cardiotomy path and the venous path. These differences produce the above-mentioned problems.